Glyphosate works wonders as does Round Up. Glyphosate is cheaper for the application you are looking for. Personally I would do a soil test after you kill the weeds and brush and till if you choose to. 40 acres is a lot of area for deer to roam. I like keeping small travel plots throughout the property so deer feel right at home. Check out some of our progress on our food plots at HuntingEastCoast.com